I do not want to hurt him, how can I let him down easy?

I've been seeing/ "dating" my friend & classmate for about a month and a half now. In the beginning I wasn't so sure, I thought I could grow to like him but I've come to realize that we were meant to stay friends. He recently mentioned getting more serious and I know I cannot commit, but I don't know how to handle this situation. I don't want to hurt him, and I don't want to bare the awkwardness of facing him everyday in class. We kept everything secret from our classmates as well. What kinds of things could I possibly say to help ease the situation?

I know we have less in common than he thinks, and I don't feel the sparks when he's around the way I thought I could.